Ischemia detection using a heart sound sensor

ABSTRACT

A system comprising an implantable medical device (IMD) includes an implantable heart sound sensor to produce an electrical signal representative of at least one heart sound. The heart sound is associated with mechanical activity of a patient&#39;s heart. Additionally, the IMD includes a heart sound sensor interface circuit coupled to the heart sound sensor to produce a heart sound signal, and a signal analyzer circuit coupled to the heart sound sensor interface circuit. The signal analyzer circuit measures a baseline heart sound signal, and deems that an ischemic event has occurred using, among other things, a measured subsequent change in the heart sound signal from the established baseline heart sound signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 11/148,107 filed Jun. 8, 2005, which isincorporated herein by reference in its entirety.

This application is related to the following co-pending, commonlyassigned U.S. patent application Ser. No. 10/900,570 entitled“DETERMINING A PATIENT'S POSTURE FROM MECHANICAL VIBRATIONS OF THEHEART,” filed on Jul. 28, 2004, Ser. No. 10/703,175, entitled “A DUALUSE SENSOR FOR RATE RESPONSIVE PACING AND HEART SOUND MONITORING,” filedon Nov. 6, 2003, Ser. No. 10/334,694 entitled “METHOD AND APPARATUS FORMONITORING OF DIASTOLIC HEMODYNAMICS,” filed on Dec. 30, 2002, Ser. No.10/746,874 entitled “A THIRD HEART SOUND ACTIVITY INDEX FOR HEARTFAILURE MONITORING,” filed on Dec. 24, 2003, Ser. No. 11/037,275,entitled “METHOD FOR CORRECTION OF POSTURE DEPENDENCE ON HEART SOUNDS,”filed on Jan. 18, 2005, Ser. No. 60/631,742 entitled “CARDIAC ACTIVATIONSEQUENCE MONITORING FOR ISCHEMIA DETECTION,” filed on Nov. 30, 2004, andSer. No. 11/129,050, entitled “METHOD AND APPARATUS FOR CARDIACPROTECTION PACING,” filed on May 16, 2005, each of which is herebyincorporated by reference.

TECHNICAL FIELD

The field generally relates to implantable medical devices and, inparticular, but not by way of limitation, to systems and methods formonitoring the mechanical functions of the heart.

BACKGROUND

Implantable medical devices (IMDs) are devices designed to be implantedinto a patient. Some examples of these devices include cardiac functionmanagement (CFM) devices such as implantable pacemakers, implantablecardioverter defibrillators (ICDs), cardiac resynchronization devices,and devices that include a combination of such capabilities. The devicesare typically used to treat patients using electrical therapy and to aida physician or caregiver in patient diagnosis through internalmonitoring of a patient's condition. The devices may include electrodesin communication with sense amplifiers to monitor electrical heartactivity within a patient, and often include sensors to monitor otherinternal patient parameters. Other examples of implantable medicaldevices include implantable diagnostic devices, implantable insulinpumps, devices implanted to administer drugs to a patient, orimplantable devices with neural stimulation capability.

Heart sounds are associated with mechanical vibrations from activity ofa patient's heart and the flow of blood through the heart. Heart soundsrecur with each cardiac cycle and are separated and classified accordingto the activity associated with the vibration. The first heart sound(S1) is the vibrational sound made by the heart during tensing of themitral valve. The second heart sound (S2) marks the beginning ofdiastole. The third heart sound (S3) and fourth heart sound (S4) arerelated to filling pressures of the left ventricle during diastole.Heart sounds are useful indications of proper or improper functioning ofa patient's heart. The present inventors have recognized a need forimproved sensing of events related to cardiac activity.

SUMMARY

This document discusses, among other things, systems and methods formonitoring mechanical functions of the heart. A system embodimentincludes an implantable medical device (IMD). The IMD includes animplantable heart sound sensor operable to produce an electrical signalrepresentative of at least one heart sound. The heart sound isassociated with mechanical activity of a patient's heart. Additionally,the IMD includes a heart sound sensor interface circuit coupled to theheart sound sensor to produce a heart sound signal, and a signalanalyzer circuit coupled to the heart sound sensor interface circuit.The signal analyzer circuit is operable to measure a baseline heartsound signal, and to deem that an ischemic event occurred using ameasured subsequent change in the heart sound signal from theestablished baseline heart sound signal.

A method embodiment includes sensing a baseline heart sound signal usingan implantable medical device (IMD), sensing at least one subsequentheart sound signal for the patient using the IMD, and deeming that anischemic event occurred using at least a measured change in at least oneheart sound signal from the baseline heart sound signal.

This summary is intended to provide an overview of the subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the subjectmatter of the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of progression of heart failure.

FIG. 2 illustrates an embodiment of portions of a system that uses animplantable medical device.

FIG. 3 shows portions of an embodiment of a system for monitoring heartsounds.

FIGS. 4A-C illustrate heart sound signals obtained from an animal study.

FIG. 5 illustrates an embodiment of an implantable medical devicecoupled by leads to a heart.

FIG. 6 shows portions of an embodiment of a system for monitoring heartsounds and electrocardiograms.

FIG. 7 is a block diagram of a method of detecting ischemia using aheart sound sensor.

FIG. 8 is a block diagram of another embodiment of a method of detectingischemia using a heart sound sensor.

FIG. 9 is a block diagram of another embodiment of a method of detectingischemia using a heart sound sensor.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and specific embodimentsin which the invention may be practiced are shown by way ofillustration. It is to be understood that other embodiments may be usedand structural or logical changes may be made without departing from thescope of the present invention.

Because heart sounds are a mechanical measure of a patient's hemodynamicsystem, monitoring of heart sounds aids caregivers in detecting overallprogression of heart disease. For example, for detection of ischemia, anincrease in ventricular chamber stiffness and an increase in the degreeof restrictive filling are correlated to an increase in loudness of S3heart sounds. Conversely, because ischemia is associated with a decreasein ventricular chamber contractility, ischemia is correlated to adecrease in the loudness of the S1 heart sound. When S3 heart sounds arepresent in patients experiencing acute chest pain, such patients arebelieved to have an increased likelihood of myocardial infarction overother causes of chest pain.

An acute myocardial infarction (AMI) is a complete occlusion of acoronary artery. It is typically caused by a rupture of plaque in anarrowed artery and results in a pronounced change in a patient'shemodynamic system. When at least twenty-five percent of the leftventricle becomes acutely ischemic, the end-diastolic pressure and theend-diastolic volume increase which results in increased loudness of S3heart sounds, S4 heart sounds, or both S3 and S4 heart sounds, dependingon the condition of the heart.

FIG. 1 illustrates an example of changes in diastolic pressure withworsening heart failure (HF). The Figure shows graphs 110 of left atrium(LA) and left ventricle (LV) diastolic pressures and graphs 120 of theassociated E-wave and A-wave. An E-wave refers to the peak mitral flowvelocity during passive filling, and an A-wave refers to the peak mitralflow velocity during atrial contraction. The graphs 100 represent stagesof worsening HF from a normal case on the left to irreversible flowrestriction on the right. As the HF condition worsens, the filling timeshortens and the left ventricular pressure (LVP) and left atrialpressure (LAP) become more elevated. Depending on the condition of theheart, this results in increased loudness of S3 heart sounds, S4 heartsounds, or both S3 and S4 heart sounds. If a baseline of heart soundmeasurements for a patient is established, a change from the baseline isa good indication that something has changed in the hemodynamic systemof the patient.

The progression of HF is typically accompanied by changes in heartsounds over time. First, an S4 heart sound may develop while the heartis still relatively healthy. Second, the S4 heart sound becomes morepronounced. Third, as deterioration of the left ventricle continues, S3heart sounds become more pronounced. Sometimes, this is accompanied by adecrease in S1 heart sounds due to a decrease in the heart's ability tocontract. Thus, ongoing or continuous monitoring of heart sounds wouldgreatly assist caregivers in monitoring heart disease. However,individual patients may exhibit unique heart sounds that complicate ageneralized approach to heart sound monitoring. For example, the merepresence of an S4 heart sound is not necessarily indicative of heartdisease because normal patients may have an S4 heart sound. Anothercomplication develops if a patient experiences atrial fibrillation whenan ischemia occurs. In this case a strong atrial contraction, and theassociated S4 heart sound, is likely to be absent due to the atrialfibrillation. This results in an increase in the S3 heart sound withoutan associated S4 heart sound or without an increase in an S4 heartsound. Therefore, the progression of heart disease, such as HF and anischemic event, is typically better monitored by establishing apatient-specific control baseline heart sound measurement and thenmonitoring for changes from that baseline. The baseline could beestablished in one or several different criteria, such as at particularphysiologic or pathophysiologic state, at a specific posture, at aparticular time of day, etc.

Changes due to AMI are immediate and result in a heart sound changewithin seconds or minutes. In contrast, heart sound changes due toworsening HF are gradual and occur over hours or days. Therefore, notjust the change but the timeframe of the occurrence of the change inheart sounds can be used to detect overall progression of heart disease.Additionally, relationships between heart sounds can be used todetermine the likelihood of an ischemic event. For example, the dynamicsbetween the S3 and S4 heart sounds with respect to the HF progressioncan be used to determine the likelihood that a patient experienced anischemic event. An appearance of the S3 and S4 heart sounds is morelikely to indicate a recent occurrence of an ischemic event if the S4/S3ratio is high than if the S4/S3 ratio is low, which would insteadindicate that a patient is in a more advanced stage of HF.

Implantable medical devices (IMDs) can include sensors to monitorinternal patient parameters such as heart sounds. Typically, the heartsound sensor is an accelerometer monitoring the vibrations associatedwith heart sounds. Because the devices are implantable, they can be usedto provide ongoing or continuous ambulatory monitoring of a patient'sheart sounds. The implanted device is used to first establish a baselinefor its individual patient during a pre-AMI period. The device thenmonitors the patient heart sounds to detect changes from the baseline.

FIG. 2 illustrates an embodiment of a system 200 that uses an IMD 210.The system 200 shown is one embodiment of portions of a system 200 usedto treat a cardiac arrhythmia or otherwise improve heart function. Apulse generator (PG) or other MD 210 is coupled to a heart 205 of apatient 202 by a cardiac lead 208, or additional leads. A leadless IMDis also possible. Examples of IMD 210 include, without limitation, apacer, a defibrillator, a cardiac resynchronization therapy (CRT)device, or a combination of such devices. Other examples includeimplantable diagnostic devices, a drug pump, and a neural stimulationdevice. System 200 also typically includes an IMD programmer or otherexternal system 270 that provides wireless communication signals 260 tocommunicate with the IMD 210, such as by using radio frequency (RF) orother telemetry signals.

In FIG. 2, cardiac lead 208 includes a proximal end that is coupled toIMD 210 and a distal end, coupled by an electrode or electrodes to oneor more portions of a heart 205. The electrodes typically delivercardioversion, defibrillation, pacing, resynchronization therapy, orcombinations thereof to at least one chamber of the heart 205. IMD 210includes components that are enclosed in a hermetically-sealed canisteror “can.” Additional electrodes may be located on the can, or on aninsulating header, or on other portions of MD 210, such as for providingunipolar pacing and/or defibrillation energy, for example, inconjunction with the electrodes disposed on or around heart 205. Thelead 208 or leads and electrodes are also typically used for sensingelectrical activity of a heart 205, or other.

Implantable heart sound sensors are generally implantable acousticsensors that convert the detected sounds of the heart into an electricalsignal representative of the heart sounds. Typically, an acoustic sensorfor an IMD 210 includes an accelerometer mounted within the can. Inanother sensor example, a microphone is located within the can. Inanother example, the sensor includes a strain gauge.

FIG. 3 shows portions of an embodiment of a system 300 for monitoringheart sounds. The system 300 includes an implantable medical device(IMD) 305. The IMD 305 includes an implantable heart sound sensor 310, aheart sound sensor interface circuit 320 coupled to the heart soundsensor 310, and a signal analyzer circuit 330 coupled to the heart soundsensor interface circuit 320. The heart sound sensor 310 produces anelectrical signal representative of at least one heart sound. The heartsound sensor interface circuit 320 provides a heart sound signal to thesignal analyzer circuit 330. The signal analyzer circuit 330 measures abaseline heart sound signal, such as by being operable to perform analgorithm or algorithms implemented by hardware, software, firmware orany combination of hardware, software or firmware. In some examples, thebaseline heart sound signal is an aggregate of a patient's differentheart sound signals that occur during a cardiac cycle. In anotherexample, the baseline signal represents a subset of the heart sounds,such as one type of heart sound. In one example, the baseline heartsound signal is established for an individual patient during a pre-AMIperiod. In another example, the baseline is established at time ofimplant. In yet another example, the baseline is established while apatient is in a predetermined physiologic or pathophysiologic state.

In some examples, the signal analyzer circuit 330 includes an averagingcircuit and establishes a baseline heart sound signal by forming anensemble or other average of multiple sampled values of like heart soundsignals. One example of descriptions of systems and methods forobtaining ensemble averages of heart sound signals is found in thecommonly assigned, co-pending U.S. patent application Ser. No.10/746,874 by Siejko et al., entitled “A Third Heart Sound ActivityIndex for Heart Failure Monitoring,” filed on Dec. 24, 2003, which isincorporated herein by reference. In some examples, the signal analyzercircuit 330 includes a low pass filtering circuit. In some examples, thesignal analyzer circuit 330 includes a central tendency circuit andestablishes a baseline signal by determining the central tendency ofmultiple sampled values of the heart sound signals. The baseline signalis stored in memory in, or coupled to, the signal analyzer circuit 330.In some examples, the baseline signal is loaded into the memory using anexternal device to communicate with the IMD 305. After the baseline isestablished, the signal analyzer circuit 330 monitors one or more heartsound signals for any change from the baseline signal. In some examples,the baseline signal is an aggregate of a patient's different heart soundsignals and the change includes a change from that aggregate of signals.In some examples, the baseline signal represents a subset of the heartsounds that occur during a cardiac cycle, such as one type of heartsound, and the change includes a change in the one type of heart soundfrom the baseline heart sound. In some examples, a sampled segment ofthe heart sound signal that includes the change is stored in the memory.Some examples of the sampled segment include a segment sampled beforethe change occurred and a segment sampled after the change occurred.Upon a particular of change, the signal analyzer circuit 330 deems thata patient has experienced an ischemic event, such as an acute myocardialinfarction.

Once the signal analyzer 330 deems that such an event has occurred, thisinformation can then be used by the signal analyzer circuit 330 toprovide an indication of the event. In one example, the signal analyzercircuit 330 activates an alarm, such as a buzzer or other audibleindication in the IMD 305, to indicate that an ischemic event occurred.In another example, the IMD 305 includes a communication circuit coupledto the signal analyzer circuit 330 and the MD 305 communicatesinformation about the measured change from the baseline in the heartsound signal to an external device. In some examples, the externaldevice initiates measurements of the heart sound signals. In someexamples, the MD 305 transmits the signal segment stored in memory tothe external device. In some examples, the external device is an IMDprogrammer and the IMD 305 indicates that an ischemic event has occurredby setting a status indication readable by the programmer upon asubsequent interrogation of the IMD 305. In another example, theexternal device is a repeater that retransmits information from the IMD305 over a network. In some examples, the external device is incommunication with the MD 305 and a computer network such as a hospitalnetwork or the Internet. The indication of the ischemic event or analarm can then be transmitted to a caregiver using the network. Anindication or alarm provided to the patient has further uses, such as todirect the patient to take a drug, adjust medication, or to seekimmediate medical assistance.

Ischemic events can be detected from a variety of different measuredchanges from one or more baseline heart sound signals. An example isshown in FIGS. 4A-4C, which illustrate heart sound signals obtained froman animal study. FIG. 4A shows an example of a baseline heart soundsignal 410 obtained for an animal that was measured using an implantedaccelerometer-type heart sound sensor. The signal 410 includes the S1heart sound 415 and the S2 heart sound 420. FIG. 4B shows an example ofa measured heart sound signal 425 after ischemia was induced in theanimal by micro-embolization. The signal 425 includes the S1 heart sound430, an S2 heart sound 435, and further shows the presence of the S3heart sound 440 and the S4 heart sound 445. FIG. 4C shows an example ofa heart signal 450 after the animal decompensated. FIG. 4C shows thatthe amplitude of the S3 heart sound 455 has increased from FIG. 4B, andthe amplitude of the S3 heart sound has changed relative to the S1 andS2 heart sounds. FIGS. 4A-C show that in some examples, an ischemicevent results in the presence of at least one heart sound in themeasured signal 425, 450, such as the S3 or S4, which is absent from thebaseline heart sound signal 410. In some examples, an ischemic eventresults in a reduction in amplitude of at least one heart sound, such asthe S1 or S2 heart sound, in the measured signal 425, 450 compared tothe baseline heart sound signal 410. In some examples, the signalanalyzer circuit 330 deems that an ischemic event has occurred from theduration of an occurrence of a heart sound, such as the S3 or S4 heartsound, that is absent from the baseline heart sound signal 410. In someexamples, the signal analyzer circuit 330 deems that an ischemic eventhas occurred from the frequency of re-occurrence of a heart sound, suchas the S3 or S4 heart sound, that is absent from the baseline heartsound signal 410.

In some system examples, the signal analyzer circuit 330 deems that anischemic event has occurred by measuring a change in amplitude of themeasured heart sound signal from the amplitude of a baseline heart soundsignal. The signal analyzer circuit 330 measures the amplitude of aparticular type of heart sound, such as the S1, S2, S3, or the S4 heartsound, and compares the measured amplitude to its corresponding baselineamplitude. However, not all patients will exhibit an S3 or S4 heartsound substantially all or most of the time. In some examples, themeasured change is deemed to result from an ischemic event if themeasured change is a specified increase in amplitude of an S3 or S4heart sound from the corresponding baseline amplitude. In some examples,if the change in amplitude is at least a specified percentage increasefrom the corresponding baseline amplitude, an ischemic event is deemedto have occurred. In some examples, the measured change is deemed toresult from an ischemic event if the measured change is a specifieddecrease in amplitude of an S1 or S2 heart sound from the correspondingbaseline amplitude.

In some examples, the signal analyzer circuit 330 establishes a baselineamplitude of a first heart sound normalized with respect to a secondheart sound. Such normalization includes calculating a ratio between atleast two different heart sounds associated with the same cardiac cycle.The signal analyzer circuit 330 deems that an ischemic event occurredwhen it measures a change in the ratio from a ratio of the correspondingbaseline amplitude, such as, for example, a percentage increase in theratio from the corresponding baseline ratio. In one example, theamplitude of the S3 or S4 heart sound is normalized with respect to theamplitude of the S1 or S2 heart sound. Because some ischemic events areassociated with both an increase in the amplitude of the S3 or S4 heartsound and a decrease of the S1 or S2 heart sound, an advantage of usingnormalization is that it is more sensitive to some types of ischemicchanges than an amplitude measurement alone. In another example, theamplitude of the S4 heart sound is normalized with respect to the S1heart sound. In another example, the amplitude of the S4 heart sound isnormalized with respect to the S3 heart sound to provide an indicationof the relative changes between the S3 and S4 heart sound. If thenormalization includes a ratio of the heart sounds (S4/S3), in the earlystages of heart disease the ratio increases with the increase in the S4heart sound. Later, the ratio decreases due to the later increase in theS3 heart sound. In some examples, the amplitude of the S1, S3, or S4heart sound is normalized with respect to the amplitude of the S2 heartsound.

In some examples, the signal analyzer circuit 330 includes a frequencyanalyzer circuit. This permits the signal analyzer circuit 330 toestablish a baseline power spectrum of a heart sound, such as the S1,S2, S3, or S4 heart sound, by calculating the power in the heart soundsignal in several frequency bands. The signal analyzer circuit 330 deemsthat an ischemic event has occurred by measuring a change in the powerspectrum of a heart sound signal from the corresponding baseline powerspectrum. In some examples, the signal analyzer circuit 330 deems thatan ischemic event occurred using a heart sound amplitude measurementthat is normalized with respect to the calculated power in the signal.In the normalization, a power of the heart sound signal is calculated inseveral frequency bands. The calculated power of a specific portion orportions of the signal is used to normalize the amplitude. As anexample, the power calculated from a high frequency band of a heartsound signal sensed during systole can be used to normalize theamplitude of the heart sound.

In some examples, the IMD 305 includes a posture sensor, and the IMD 305measures heart sound signals in association with a posture of a patient.For example, heart sound signals are only measured or used while thepatient is in a particular posture (e.g., upright), or measurements madewhile the patient is in one posture (e.g., upright) are distinguishedfrom measurements made while the patient is in another posture (e.g.,lying down). This removes a source of variability of the heart soundsignals due to patient posture. A description of systems and methods formonitoring heart sounds in association with patient posture are found incommonly assigned, co-pending U.S. patent application Ser. No.11/037,275 by Siejko et al., entitled “Method for Correction of PostureDependence on Heart Sounds,” filed on Jan. 18, 2005, which isincorporated herein by reference.

In some examples, the IMD 305 measures heart sounds in association withpatient activity. To detect patient activity, some examples of the IMD305 include a patient activity sensor. If the heart sound sensor is anaccelerometer, the activity sensor can be a second accelerometer or thesame accelerometer as the heart sound sensor. In some examples, the MD305 infers the patient's activity level from a patient's heart rate,respiration rate, or minute ventilation such as by using a thoracicimpedance sensor. In some examples, the IMD 305 deems that an ischemicevent has occurred using a measured change in the heart sound signalfrom at least one corresponding baseline heart sound signal specificallyestablished for exercise conditions. An early or mid-diastolic mergingof S3 and S4 heart sounds (i.e., a gallop rhythm) developing afterexercise is believed to nearly always signify myocardial disease withreduced myocardial function. Thus, in some examples, an ischemic eventcan be detected by the presence of the gallop rhythm that is absent fromthe baseline heart sound signal.

In some examples, the signal analyzer circuit 330 uses the temporalnature of changes in one or more heart sounds to distinguish between anischemic event and an indication of a worsening condition of HFdecompensation. For example, if the measured change from the baselineheart sound signal occurs relatively suddenly with a time constant thatranges from a few seconds to a few minutes (e.g., five minutes), theepisode is deemed to be an ischemic event. If the measured change fromthe baseline heart sound signal occurs with a time constant of severalhours (e.g., four hours) or days, the episode is deemed to be anindication of a worsening condition of HF decompensation. In someexamples, the signal analyzer circuit 330 activates a same or differentalarm to indicate the worsening condition of congestive heart failure.In some examples, the signal analyzer circuit 330 stores data associatedwith measured changes from the baseline signal in memory. The signalanalyzer circuit 330 is capable of calculating trend data of themeasured changes and storing the trend data in memory. In some examples,the signal analyzer circuit 330 uses the trend data to generate acongestive heart failure status indication. In some examples, the IMD305 transmits the trend data to an external device, such as for display.

FIG. 5 illustrates an IMD 510 coupled to heart 505, such as by one ormore leads 508A-B. Heart 505 includes a right atrium 500A, a left atrium500B, a right ventricle 505A, a left ventricle 505B, and a coronary vein520 extending from right atrium 500A. In this embodiment, atrial lead508A includes electrodes (electrical contacts, such as ring electrode525 and tip electrode 530) disposed in, around, or near an atrium 500Aof heart 505 for sensing signals, or delivering pacing therapy, or both,to the atrium 500A. Lead 508A optionally also includes additionalelectrodes, such as for delivering atrial cardioversion, atrialdefibrillation, ventricular cardioversion, ventricular defibrillation,or combinations thereof to heart 505. Lead 508A optionally furtherincludes additional electrodes for delivering pacing orresynchronization therapy to the heart 505.

Ventricular lead 508B includes one or more electrodes, such as tipelectrode 535 and ring electrode 540, for sensing signals, fordelivering pacing therapy, or for both sensing signals and deliveringpacing therapy. Lead 508B optionally also includes additionalelectrodes, such as for delivering atrial cardioversion, atrialdefibrillation, ventricular cardioversion, ventricular defibrillation,or combinations thereof to heart 505. Such electrodes typically havelarger surface areas than pacing electrodes in order to handle thelarger energies involved in defibrillation. Lead 508B optionally furtherincludes additional electrodes for delivering pacing orresynchronization therapy to the heart 505.

Other forms of electrodes include meshes and patches which may beapplied to portions of heart 505 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by IMD510. In one embodiment, one of atrial lead 508A or ventricular lead 508Bis omitted, i.e., a “single chamber” device is provided, rather than thedual chamber device illustrated in FIG. 5. In another embodiment,additional leads are provided for coupling the IMD 510 to other heartchambers and/or other locations in the same heart chamber as one or moreof leads 508A-B. The present methods and systems will work in a varietyof configurations and with a variety of electrical contacts or“electrodes,” including a leadless system that uses electrodes remotefrom, rather than touching, the heart 505.

FIG. 6 shows portions of an example of a system 600 for monitoring heartsounds and electrical cardiac signals. In this example, the system 600includes an IMD 605 and an external device 610 operable to communicatewith the IMD 605. The IMD 605 includes a heart sound sensor 625 and aheart sound sensor interface circuit 620 coupled to a signal analyzercircuit 630. The heart sound sensor 625 produces electrical signalsrepresentative of at least one heart sound. The IMD 605 also includes amemory circuit 635, a cardiac signal sensing circuit 640, and a therapycircuit 645.

The therapy circuit 645 is coupled to one or more electrodes. In oneexample, the therapy circuit 645 is attached to a cardiac lead or leadssuch as to provide cardioversion, defibrillation, pacing,resynchronization therapy, or one or more combinations thereof to atleast one chamber of the heart. The memory circuit 635 stores heartsound measurements. In some examples, the memory circuit 635 also storessegments of measured cardiac signals. The IMD 605 further includes acommunication circuit 650. The external device 610 communicateswirelessly with the IMD 605 by using RF or other telemetry signals. TheIMD 605 communicates heart sound information to the external device 610.In some examples, the external device 610 is part of, or is incommunication with, a computer network such as a hospital computernetwork or the Internet.

The cardiac signal sensing circuit 640 senses electrical cardiac signalsassociated with the action potential signals of a heart. The actionpotentials propagate through the heart's electrical conduction system toexcite various regions of myocardial tissue. The sensing circuit 640provides an electrical signal representative of such signals. Examplesof cardiac signal sensing circuits 640 include, without limitation, asubcutaneous electrocardiogram (ECG) sensing circuit, an intracardiacelectrogram (EGM) sensing circuit, and a wireless ECG sensing circuit.In a subcutaneous ECG sensing circuit, electrodes are implanted beneaththe skin and the ECG signal obtained is referred to as subcutaneous ECGor far-field electrogram. In an intracardiac EGM circuit, at least oneelectrode is placed in or around the heart. A wireless ECG includes aplurality of electrodes to provide differential sensing of cardiacsignals to approximate a surface ECG. Descriptions of wireless ECGsystems are found in commonly assigned, co-pending U.S. patentapplication Ser. No. 10/795,126 by McCabe et al., entitled “Wireless ECGin Implantable Devices,” filed on Mar. 5, 2004, which is incorporatedherein by reference.

The signal analyzer circuit 630 measures the heart sound signal and thecardiac signal. In some embodiments, the signal analyzer circuit 630measures the heart sounds in correspondence with a sensed heartdepolarization, such as to help identify particular heart sounds. Thedesired heart sound is identified by aligning the heart sound signal toknown features in a sensed cardiac signal. For example, an R-wave sensedby the cardiac signal sensing circuit 640 helps align S1 and S2 heartsounds sensed with the heart sound sensor 625. This is useful for, amongother things, identifying a time window associated with a particularheart sound, such as when establishing a baseline heart sound signal.

In some examples, the IMD 605 deems that an ischemic event occurredusing either the cardiac signal or the heart sound signal. In someexamples, the IMD 605 deems that an ischemic event occurred using both ameasured change in the cardiac signal from an established baselinecardiac signal and a measured change in the heart sound signal from anestablished corresponding baseline heart sound signal. Using bothsignals to conclude that an ischemic event occurred increases theconfidence or specificity in the conclusion. As an example, the signalanalyzer circuit 630 deems that an ischemic event has occurred using aspecified measured minimum change from the corresponding baseline heartsound signal and a sensed cardiac signal having an S-wave to T-wave(“ST”) interval that deviates by at least a specified amount from an STinterval of a baseline cardiac signal. In another example, the signalanalyzer circuit 630 deems an ischemic event to have occurred upondetecting at least a specified measured change in the heart sound signaland a sensed cardiac signal having a T-wave that is inverted from theT-wave in the baseline cardiac signal. In another example, the signalanalyzer circuit 630 deems an ischemic event to have occurred upondetecting at least a specified measured change in the heart sound signaland a sensed cardiac signal having a sensed T-wave that is biphasicrelative to a monophasic T-wave in the baseline cardiac signal.Descriptions of systems and methods for detecting ischemia usingwireless ECG circuits are found in commonly assigned, co-pending U.S.Patent Application Ser. No. 60/631,742 by Zhang et al., entitled“Cardiac Activation Sequence Monitoring for Ischemia Detection,” filedon Nov. 30, 2004, which is incorporated herein by reference.

In some examples, a surface ECG sensing circuit is coupled to theexternal device. The ECG sensing circuit includes at least twoelectrodes attached to the skin of a patient to sense cardiac signals.The external device then deems that an ischemic event occurred usingboth a measured change in an ECG signal obtained from the external ECGcircuit and a measured change in the heart sound signal obtained fromthe IMD 605.

In some examples, the signal analyzer circuit 630 deems that an ischemicevent has occurred using a temporal relationship between the measuredchange in the heart sound signal and the sensed event indicated by thecardiac signal. In one such example, the signal analyzer circuit 630deems an ischemic event to have occurred by detecting a sequence in timeof heart sound signal changes and cardiac signal changes. As anillustrative example of such a sequence of events, the signal analyzercircuit 630 may first measure a decrease in the S1 heart sound signalfrom the S1 baseline. The signal analyzer circuit 630 subsequentlymeasures a deviation in the ST interval of the cardiac signal from thebaseline cardiac signal. Later, an S3 heart sound appears on themeasured heart sound signal, where the S3 heart sound is absent in thebaseline heart sound signal. When such a sequence occurs, the signalanalyzer circuit 630 deems that an ischemic event has occurred. Asanother example, the cardiac signal T-waves discussed above typicallydisappear after about one hour following an ischemic event, but theevidence from heart sound changes remains. In one example, the signalanalyzer circuit 630 deems that an ischemic event has occurred when thecardiac signal feature appears temporarily (e.g., for a time period ofless than about one hour), but the heart sound signal changes frombaseline persist, even after the temporary cardiac signal featuresubsides.

In some examples, the signal analyzer circuit 630 uses rules to combinethe outputs of the cardiac sensing circuit 640 and the heart soundsensor circuit 620. In one example, the signal analyzer circuit 630assigns at least a first weight to the measured change from baseline inthe heart sound signal and assigns at least a second weight to a sensedevent indicated by the change in the ECG signal. The signal analyzercircuit 630 then deems an ischemic event to have occurred according toat least one rule incorporating the measured change in the heart soundsignal, the sensed event indicated by the ECG signal and the assignedweights. Table 1 below shows an example where the rule applied is adecision matrix. The signal analyzer circuit 630 applies a low, medium,or high weight to the strength of a measured S4 heart sound change.Similarly, the signal analyzer circuit 630 applies a low, medium, orhigh weight to a measured deviation in an ST interval in an ECG signal.In one example, the weights are applied based on amplitude changes froma corresponding patient-specific baseline.

TABLE 1 ST deviation High — — High Confidence Level Medium — — — Low LowConfidence Level — — S4 Heart Low Medium High Sound

If the weights of the measured signals are both low, the signal analyzercircuit 630 has a low confidence level that an ischemic event occurred.If the weights of the measured signals are both high, the signalanalyzer circuit 630 has a high confidence level that an ischemic eventoccurred. The rest of the decision matrix can be programmed based onfactors such as history of the patient or experience of the caregiver.

In some examples, the IMD 605 includes one or more other sensors, suchas to measure intracardiac or trans-thoracic impedance, or bloodpressure. In some examples, the signal analyzer circuit 630 uses atleast one rule to blend the outputs of the various sensors to make adecision as to whether a patient has experienced an ischemic event. Insome examples, the signal analyzer circuit 630 assigns weights tocorresponding outputs of the sensors, and applies at least one rule tomerge the sensor outputs and the measured change in the heart soundsignal using the weights. The signal analyzer circuit 630 determineswhether an ischemic event occurred based on the application of the rule.In some examples, the signal analyzer circuit 630 applies one or morefuzzy logic rules that use the weights to merge the sensor outputs andthe measured change in the heart sound signal to determine whether anischemic event occurred.

In some examples, the signal analyzer circuit 630 discriminates atransient ischemic event from an AMI event using a measured change in aheart sound signal from an established baseline heart sound signal.Sometimes an ischemic event detectable by the system 600 may be atransient ischemic. Transient ischemic events can occur in non-emergencysituations, such as a result of exercise for example. In one systemexample, the signal analyzer circuit 630 discriminates a transientischemic event from an AMI event based on the duration of the measuredsubsequent change from the established heart sound signal. As anillustrative example, the signal analyzer circuit 630 deems an ischemicevent a transient event when a heart sound (such as an S3 or S4 heartsound, or a combination of S3 and S4 heart sounds) not present in theestablished baseline signal briefly appears and then disappears from theheart sound signal. In another system example, the signal analyzercircuit 630 discriminates a transient ischemic event from an AMI eventbased on a change in amplitude of a heart sound. As an illustrativeexample, the signal analyzer circuit 630 may deem that a change inamplitude is a transient ischemic event if the amplitude change is belowa predetermined threshold change value and an AMI event if the change isabove the change value.

In yet another system example, the signal analyzer circuit 630discriminates a transient ischemic event from an AMI event based on atemporal relationship of a sensed event indicated by a sensed cardiacsignal and the measured subsequent change in the heart sound signal fromthe established baseline heart sound signal. For example, if the signalanalyzer circuit 630 detects a change in both the cardiac signal and theheart sound signal, but the cardiac signal change goes away while theheart sound change remains, the signal analyzer circuit 630 deems theevent an AMI event. If the signal analyzer circuit 630 detects a changein the cardiac signal but the heart sound signal does not change, thesignal analyzer circuit 630 deems the event a transient ischemic event.In some examples of the system 600, an indication of AMI events or bothindications of AMI events and transient ischemic events are stored inthe memory 635, communicated to the external device 610, or both storedin the memory 635 and communicated to the external device 610.

FIG. 7 is a block diagram of a method 700 of detecting ischemia using aheart sound sensor. At 710, a baseline heart sound signal is sensedusing an IMD. The heart sound or sounds are associated with mechanicalvibrations of a heart of a patient. At 720, at least one subsequentheart sound signal for the patient is sensed using the IMD. In someexamples of the method, the baseline heart sound signal and subsequentheart sound signal are sensed in association with a posture of a patientto remove the variability of the heart sound signal measurements withpatient posture. At 730, an ischemic event is deemed to have occurredusing at least a measured change in at least one heart sound from thecorresponding baseline heart sound signal. In some examples, the heartsounds are continuously, or occasionally, monitored for changes from thecorresponding baseline.

According to some examples, an occurrence of an ischemic event isinferred from a measured increase in amplitude of a heart sound from abaseline heart sound amplitude. In some examples, the baseline includessampled amplitudes stored in a memory. In some examples, the ischemicevent is inferred using a measured decrease in amplitude of a heartsound from a baseline heart sound amplitude. In some examples, theischemic event is inferred using a measured increase in amplitude of afirst heart sound and a measured decrease in a second heart sound. Insome examples, a normalization of a first heart sound with respect to asecond heart sound is monitored. As an example, an occurrence of anischemic event is inferred from an increase in the ratio measuredrelative to a baseline value of the S3/S1 ratio. As another example, theratio of the S4 amplitude to the S3 amplitude is monitored. Anoccurrence of an ischemic event is inferred from an increase in theratio measured relative to a baseline value of the S4/S3 ratio. In yetanother example, the amplitude of the S1, S3, or S4 heart sound isnormalized with respect to the S2 heart sound.

In some examples, an ischemic event is deemed to have occurred by theappearance of a transient heart sound that is missing in the baselineheart sound signal. For example, if the baseline heart sound signal hasan absence of S3 heart sounds, an ischemic event is inferred when atleast one transient S3 heart sound is detected in the signal. As anotherexample, if the baseline heart sound signal has an absence of S4 heartsounds, an ischemic event is inferred when at least one transient S4heart sound is detected in the signal. In some examples, an ischemicevent is deemed to have occurred based on the frequency with which aheart sound missing in the baseline heart sound signal appears in themeasured signals and then disappears. In some examples, an ischemicevent is deemed to have occurred based on the time duration that a heartsound missing in the baseline heart sound signal appears in the measuredsignals before it disappears. In some examples, an ischemic event isinferred when a merging of S3 and S4 heart sounds is detected in theheart sound signal. In an example, an ischemic event is inferred whenthe merging occurs at high heart rates when the time for diastole isshortened, such as at one hundred beats per minute (100 bpm) or higher.

According to some examples, frequency components of heart sounds and thepower component of the signal at the frequencies are monitored, i.e., apower spectrum is monitored. A baseline of the power spectrum of atleast one heart sound, such as the S3 or S4 heart sound, is established,such as by using a fast Fourier transform (FFT) provided by digitalsignal processing (DSP). An occurrence of an ischemic event is inferredfrom a measured change in the power spectrum of a heart sound signalfrom the corresponding baseline power spectrum. In some examples, anoccurrence of an ischemic event is inferred from the amplitude of aheart sound normalized with respect to the power in the heart soundsignal. In some examples, the method 700 further includes activating analarm to indicate that an ischemic event occurred. The alarm includes anaudible or vibrating alarm from the IMD, or an alarm from an externaldevice. The external device may receive an alarm status from the IMDwhen the IMD is next interrogated or by a communication originating fromthe IMD. The alarm may be used to notify the patient, a caregiver, orboth a patient and caregiver of the ischemic event.

According to some examples, an occurrence of an ischemic event isinferred using both a cardiac signal sensed by the IMD and the measuredchange in the heart sound signal. In some examples, the inference ismade when a measured change from established baseline signals occurs inboth the cardiac signal and the heart sound signal. In some examples, anischemic event is inferred from timing relationships between the changeor changes on the cardiac signal and the change or changes in the heartsound signals. In some examples, an ischemic event is inferred using thetemporal relationship of a sensed event indicated by the sensed cardiacsignal and the measured change in the heart sound signal from theestablished baseline heart sound signal. As an illustrative example, anischemic event is inferred when the sensed event is no longer indicatedby the sensed cardiac signal while the measured change in the heartsound signal continues to persist.

FIG. 8 is a block diagram of another embodiment of a method 800 ofmonitoring one or more mechanical functions of a heart. At 810, abaseline heart sound signal is sensed using an IMD, and at 820, at leastone subsequent heart sound signal for the patient is sensed using theIMD. At 830, an occurrence of an ischemic event is inferred if thechange in a heart sound signal from the corresponding baseline signaloccurs within a time frame of several minutes (e.g., less than fifteenminutes). This is in contrast to a change that is detected by trendingof heart sound signal measurements that indicate the change is occurringwith a time constant of several hours or days (e.g. more than fourhours). If the change from the baseline occurs over the course of hoursor days, at 840 the method 800 further includes deeming that the changeindicates worsening of a heart failure condition of the patient. In someexamples, the method 800 further includes activating the same alarm asan ischemic event alarm, or a different alarm to indicate the heartfailure condition.

FIG. 9 is a block diagram of another embodiment of a method 900 ofmonitoring one or more mechanical functions of a heart. At 910, abaseline heart sound signal is sensed using an IMD, and at 920, at leastone subsequent heart sound signal for the patient is sensed using theIMD. At 930, a transient ischemic event is discriminated from an AMIevent using a measured subsequent change in the heart sound signal fromthe baseline heart sound signal. In some method examples, discriminatinga transient ischemic event from an AMI event includes discriminatingbased on the duration of the measured subsequent change from theestablished baseline heart sound signal. A longer duration indicates AMIwhile a short duration indicates a transient event. In some examples,discriminating a transient ischemic event from an AMI event includesdiscriminating based on the measured amplitude of a subsequent changefrom the established baseline heart sound signal. A large amplitudechange is likely to indicate an AMI event, while a small amplitudechange is likely to indicate a transient ischemic event. In someexamples, discriminating a transient ischemic event from an AMI eventincludes discriminating based on a temporal relationship of a sensedevent indicated by a sensed cardiac signal and the measured subsequentchange in the heart sound signal from the established baseline heartsound signal. An event indicated by both the cardiac signal change andthe heart sound change is more likely to be an AMI event if the cardiacsignal change goes away but the heart sound change remains. An eventthat is indicated by a cardiac signal change but occurs without acorresponding change in a heart sound signal is likely to be a transientischemic change.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations, or variations, or combinations of variousembodiments. Combinations of the above embodiments, and otherembodiments not specifically described herein, will be apparent to thoseof skill in the art upon reviewing the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own.

1. A method comprising: sensing a heart sound signal using a medicaldevice, the heart sound associated with mechanical vibration of a heartof a patient; establishing a baseline value of a characteristic of atleast one of an S3 or S4 heart sound in the heart sound signal; sensingat least one subsequent heart sound signal for the patient using thedevice; deeming that an ischemic event occurred using at least ameasured subsequent change in the heart sound characteristic from theestablished baseline value of the characteristic; and providing anindication of the ischemic event to a user or process.
 2. The method ofclaim 1, wherein deeming that an ischemic event occurred using at leasta measured change includes deeming using a measured change in at leastone heart sound signal characteristic from an established baseline valueof the characteristic, the characteristic of the at least one of the S3or S4 heart sound is selected from the group consisting of: an amplitudeof the heart sound of the heart sound signal; a power spectrum of theheart sound; an amplitude of the heart sound normalized with respect toan amplitude of a second heart sound; an amplitude of the heart soundnormalized with respect to a measured power of the heart sound signalmeasured during systole; a frequency of occurrence of the heart sound inthe heart sound signal; and a duration of an occurrence of the heartsound in the heart sound signal.
 3. The method of claim 1, whereindeeming that an ischemic event occurred is inferred at least in part bydetecting a merging of S3 and S4 heart sounds in the heart sound signal.4. The method of claim 1, wherein the baseline value of heart soundcharacteristic and subsequent change in the heart sound characteristicare sensed in association with a posture of a patient.
 5. The method ofclaim 1, wherein the baseline heart sound characteristic and subsequentchange in the heart sound characteristic are sensed in association withpatient activity.
 6. The method of claim 1, wherein the baseline heartsound characteristic and subsequent change in the heart soundcharacteristic are sensed in association with an electrical cardiacsignal representative of cardiac activity of the patient.
 7. The methodof claim 1, wherein deeming that an ischemic event occurred includesongoing monitoring for any measured changes from the establishedbaseline of the heart sound characteristic.
 8. The method of claim 1,further including activating an alarm indicative of the deemed ischemicevent.
 9. The method of claim 1, wherein deeming that an ischemic eventoccurred is inferred at least in part using a measured change in theheart sound characteristic from the baseline value of the heart soundcharacteristic when a time constant of the change has a range of severalseconds to several minutes, and wherein the method includes determininga heart failure condition of a patient using the measured change in theheart sound signal from the baseline heart sound signal when the timeconstant has a range of several hours to several days.
 10. The method ofclaim 9, further including activating an alarm indicative of the heartfailure condition.
 11. The method of claim 1, wherein deeming that anischemic event occurred is inferred at least in part using both ameasured change in a cardiac signal from an established baseline cardiacsignal sensed using the device and the measured change in the heartsound characteristic.
 12. The method of claim 11, wherein deeming anischemic event occurred is inferred at least in part using a temporalrelationship between the measured change in the heart soundcharacteristic from the established baseline heart sound characteristicand the measured change in a cardiac signal from the establishedbaseline cardiac signal.
 13. The method of claim 12, wherein deeming anischemic event occurred includes using the temporal relationship of asensed event indicated by the sensed cardiac signal and the measuredchange in the heart sound characteristic from the established baselinevalue of the heart sound characteristic.
 14. The method of claim 1,wherein deeming that an ischemic event occurred includes discriminatinga transient ischemic event from an acute myocardial infarction (AMI)event using the measured change in the heart sound signal from theestablished baseline value of the heart sound characteristic.
 15. Themethod of claim 14, wherein discriminating a transient ischemic eventfrom an AMI event includes discriminating based on a duration of themeasured subsequent change from the established baseline value of theheart sound characteristic.
 16. The method of claim 14, whereindiscriminating a transient ischemic event from an AMI event includesdiscriminating based on an amplitude of the measured subsequent changefrom the established baseline value of the heart sound characteristic.17. The method of claim 14, wherein discriminating a transient ischemicevent from an AMI event includes discriminating using a sensed eventindicated by a sensed cardiac electrical signal and deeming the event anAMI event when the sensed cardiac event is no longer indicated by thesensed cardiac electrical signal while the measured change in the heartsound persists.
 18. The method of claim 14, wherein discriminating atransient ischemic event from an AMI event includes discriminating usingthe measured change in the heart sound in association with at least oneof a posture of a patient, patient activity, and a sensed cardiacelectrical signal.
 19. The method of claim 1, including measuring theheart sound in association with a sensed electrical cardiac activitysignal; assigning a first weight to the measured subsequent change inthe heart sound from the established baseline value of the heart soundcharacteristic and a second weight to a measured change in theelectrical cardiac signal from an established baseline electricalcardiac signal; and deeming an ischemic event occurred using themeasured change in the value of the heart sound characteristic, themeasured change in the electrical cardiac signal, and the assignedweights.
 20. The method of claim 1, including discriminating a transientischemic event from an acute myocardial infarction using the measuredsubsequent change in the heart sound characteristic from the establishedbaseline value of the heart sound characteristic.
 21. The method ofclaim 1, wherein sensing a heart sound signal using a medical deviceincludes sensing a heart sound signal using an implantable medicaldevice (IMD).